Spatial filtering in the detection of thermal transverse phase modulation of laser beams

We study the application of spatial filtering to the far field detection of a probe beam with transverse phase modulation induced by the refractive index distribution generated in an optically thin film by the steady state absorption of a pump beam. To this end we introduce a mathematical model able to describe the phenomenon and we study the dependence of the detected signal on the radius and the position of a circular stop operating as a spatial filter. We show, by computer calculations, that spatial filtering allows enhancement of signal/background ratio as has already been shown for the case of pulsed thermal lens spectroscopy. We give an explicit expression of the detected signal as a function of sample properties that can be used for the interpretation of measurements.